The present invention relates to the biological purification of sewage.
Known methods of biological purification of sewage almost exclusively use the active sludges system, a purification system which is time-consuming and expensive. Waste water other than sewer water, mainly industrial waste water, have in certain cases proved to be capable of purification in a more or less satisfactory manner by means of percolation in aerobic conditions through a bed of activated carbon in the pores of which colonies of bacteria are established which are capable of metabolizing or degrading the impurities contained in the water to be purified. Fundamentally this process is based on the adsorption of the impurities by the activated carbon and on the subsequent degradation of the adsorbed substances.
British Patent Specification No. 1,296,233 refers to the possibility of obtaining in this manner reductions of the chemical oxygen demand (C.O.D.) by more than 90%, with a flow of dirty water through an activated carbon bed of 12 gallon/min/foot.sup.2 (corresponding to 300 liters/hour/dm.sup.2), that is, with effective linear velocities of 30m/hour through the bed. When applying this type of process to urban sewage it has not hitherto been possible to obtain acceptable results, even when using three or more purification beds or columns in series. A certain improvement has been achieved using beds consisting of a mixture of activated carbon granules and activated sludges and continuously introducing a current of activated sludges into the flow of dirty water to be purified. With such processes, however, the whole purification plant inevitably becomes much more complicated, since it is necessary to provide a large settling tank for the recovery of the effluent sludges, as well as suitable equipment for the regeneration and recycling of such sludges.
In such known process, oxygenation air is blown into the carbon bed from below, or the dirty water is alternatively aerated immediately before its passage through the bed.